This invention relates to air cycle environment control systems that condition air and more specifically to the apparatus that cool, filter, pressurize, remove moisture and otherwise condition cabin air. The new dehumidification device uses a vapor membrane to separate moisture from gasses.
Current air cycle environmental control system (ECS) designs, especially in the application for aircraft air conditioning, have included therein 2, 3, and 4 wheel air cycle machines, with high pressure water separation cycles. All three of the ECS designs typically utilize a reheater and a condenser heat exchanger to respectively pre-cool the bleed moist air stream and then condense the water vapor in it. After condensation, the condensed water is removed by a water extractor. This water phase change, condensation, coalescence or like method, involves an enthalpy penalty due to the energy necessary to effect the transition of water from vapor to liquid phase. The general distinction among the three designs relates to the number of so-called wheels that are mechanically engaged to one another. Examples of such air cycle environment control systems are disclosed in U.S. Pat. No. 4,198,830 (issued Apr. 22, 1980), U.S. Pat. No. 5,086,622 (issued Feb. 11, 1992), U.S. Pat. No. 5,461,882 (issued Oct. 31, 1995), U.S. Pat. No. 5,887,445 (issued Mar. 30, 1999).
In the traditional air cycle environment control systems a reheater, condenser and water extractor are included for cooling and conditioning water bearing compressed air, condensing the water vapor present in it, and trapping and removing the liquid water. The resulting dehumidified air flows to the reheater where the remaining water droplets are evaporated, leaving the residual moisture in the vapor phase. Other means of removing moisture from air include vapor cycle environmental control systems that in principle is refrigeration based air cooling and water vapor condensing system, adsorption by solid desiccants, absorption by liquid desiccants, etc. An example of a water adsorption based system is disclosed in U.S. Pat. No. 5,660,048, issued Aug. 26, 1997. The invention includes a desiccant wheel for selective adsorption of water molecules. The adsorption surfaces are then regenerated by use of high temperature gas flow or a vacuum condition to complete an adsorption/desorption cycle.
These adsorption/desorption stages and associated equipment may be removed from the air conditioning system and replaced with a vapor membrane device. The water is then removed in its vapor state thereby avoiding the condensation into the liquid state. Therefore no heat of condensation is released and the efficiency of the air conditioning cycle is improved providing a thermodynamic advantage.
Other air conditioning designs include those devices that are structured to trap water vapor. U.S. Pat. No. 4,487,618, Issued Dec. 11, 1984 is an example of such a device. In this instance a wire fiber pad is used to coalesce water vapor into droplets that are then entrained by the air to be trapped as droplets for removal in the liquid state. Again such systems include the vapor to liquid state change to remove water that does not include the thermodynamic efficiency of a single vapor state extraction methodology.
As can be seen, there is a need for a less complex, improved efficiency device and method to extract water from air in air conditioning systems.
An improved dehumidification device for air cycle environment control systems according to the present invention comprises a semipermeable vapor membrane for water vapor, gas separation. In the preferred embodiment a vapor membrane device is inserted downstream of an air cycle environment control systems primary or secondary heat exchanger and upstream from the turbine. The moist air stream is passed countercurrent or cross flow relative to purge airflow on two sides of the semipermeable vapor membrane. The pressure differential between the two air streams on the feed and purge sides of the membrane causes water vapor to permeate to the purge air stream. The water vapor present in the purge air is then expelled overboard with the purge air.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.